X-ray pixel detector device and fabrication method

ABSTRACT

A method and device for producing an X-ray pixel detector, for X-ray photons, the detector presenting high efficiency combined with high resolution for obtaining a high image quality detector while at the same time minimizing the X-ray dose used. The application is particularly important whenever the X-ray photon absorption distance is much longer than the required pixel size. The arrangement presents a structure based on light-guiding of secondarily produced photons within a scintillating pixel detector in conjuction with, a CCD or a CMOS pixel detector. The structure presents a matrix ( 8 ) having deep pores ( 10 ) fabricated by high-aspect ratio silicon etching techniques producing very thin walls and with a pore spacing less or equal to the size of a pixel ( 2 ) of the image detector used. The pore matrix is subsequently filled by melting a scintillating material into the pores such that, in each pore, a single scintillating block is formed. The silicon matrix ( 8 ) may further utilize a reflective layer to increase light guiding down to the image detector chip.

TECHNICAL FIELD

The present invention relates to an X-ray pixel detector, and moreexactly to a pixel-camera based i g detector for X-ray photons with highefficiency combined with high resolution.

BACKGROUND

Silicon devices as CCDs and CMOS pixel detectors are frequently used forX-ray imaging. Due to the low stopping for X-rays in silicon, thedetector is generally coated with a scintillating layer. When usingscintillating layers for imaging there is a trade-off between quantumefficiency and resolution. In order to get high quantum efficiency forX-rays the layer should be made thick, but that will reduce the spatialresolution in the image. The quantum efficiency for X-rays is one of themost critical parameters for medical X-ray imaging devices since thesignal to noise ratio in the image depends on the number of X-rayphotons contributing to the image. Since photoelectric absorption is asingle event an X-ray photon will either be fully absorbed or passunnoticed through the detector.

X-ray generators for dental X-ray imaging operate with an acceleratingvoltage of 60-90 kV giving mean photon energy in the range 30-40 keV.The material thickness required to stop 80% of the X-ray photons is inthe range 150-500 μm for the commonly used scintillators. The primaryinteraction between the photon and the material, photoelectricabsorption, is a single event. The light in the scintillator is thengenerated by a large number of secondary reactions taking place within afew microns from the location of the primary interaction. As a result aflash of light is generated close to the spot of the primary interactionand radiated in all directions. The quantum efficiency for X-rays isthen related to the probability for the primary interaction to occur andto a very small extent to the secondary interactions. In the energyrange of interest for such an application and with the materials used asscintillators the primary interaction is generally a photoelectricabsorption. Compton scattering and other events are less likely tooccur.

The light generated in the scintillator is projected onto the sensorwith a spot size, which is proportional to the distance between thepoint of interaction and the position of absorption in the sensor. Theprojection is also affected by the refractive indexes of the materialsthe beam will pass. For a typical combination of scintillator and CCD,the scintillator thickness should be less than 100 μm to achieve aspatial resolution>10 line-pairs/mm, as required for dental X-rayimaging.

A method to improve the spatial resolution of thick scintillating layersis to define pixels in the scintillator, as proposed in EP-A2-0 534 683,U.S. Pat. No. 5,059,800 and U.S. Pat. No. 5,831,269 and to make that thelight generated within one pixel is confined within that pixel. Pixeldefinition in scintillators can be done in a number of ways, e.g.columnar growth of scintillator crystal or groove etching inscintillating films. In EP-A2-0 534 683 dicing or cutting is suggestedfor separating scintillator elements from a large scintillator block, asappropriate for larger lateral dimensions.

The method for columnar growth of scintillating crystals is well known.It has been used to grow CsI for many years. The document WO93/03496discloses for instance growth of separate columns in differentscintillators whereas in U.S. Pat. No. 4,663,187 a scintillator is heldclose to the melting point resulting in the formation of domains. Thedisadvantage of techniques for growth of separated columns is that thecolumns tend to grow together for thick layers and that light will leakto adjacent columns. It is difficult to apply a light reflector betweenthe columns.

Etching of grooves in scintillating materials is considered to beextremely difficult due to the high aspect ratios required by theapplication. With a pixel size of 50 μm and an allowed area loss of lessthan 20% the groove width should be less than 5 μm. If the filmthickness is 200 μm the aspect ratio will be 40. This aspect ratio canonly be realised by advanced silicon processing techniques whereasetching techniques for scintillating materials are far less developed.Nevertheless, U.S. Pat. No. 5,519,227 claims that laser-basedmicro-machining techniques could be used to define narrow grooves in ascintillating substrate. However, the technique is inherently slow asthe laser needs to be scanned several times in every groove.Furthermore, it is not clear whether re-deposition onto the walls willoccur as a result of this laser ablation, which could potentially blocka narrow groove.

Summarising, various techniques have been proposed for the fabricationof a scintillator array that would provide light guiding of secondaryphotons to an underlying imaging detector, These techniques are allrestricted in one or several aspects: either too large lateraldimensions (cutting, dicing), problems of forming a well-defined narrowwall (laser ablation), cross talk between adjacent pixels (columnargrowth technique) or a lengthy processing time (valid for most of thesetechniques). Finally, deposition of a reflective layer in the grooves isusually suggested to improve light guiding and reduce cross talk. But,none of these fabrication schemes have proposed a detailed scheme howthe reflective layer would be produced. This is not an easy taskconsidering the narrow pore geometry and materials involved.

Therefore there is still a desire to develop a device and it'sassociated fabrication method, which should be able to handle thickscintillating material layers but with a maintained resolution whichcorresponds to the individual pixel size. Furthermore, the fabricationtechnique should preferably be fast, as for a mass scale productiontype, and relying as much as possible on existing processes andmachinery.

SUMMARY

The objective of the present invention is to design and develop afabrication method for an X-ray pixel detector, i.e. an imaging detectorfor X-ray photons presenting high efficiency combined with highresolution to obtain a high image quality detector while at the sametime minimizing the X-ray dose used. The application is particularlyimportant whenever the X-ray photon absorption distance is much longerthan the required pixel size.

It is proposed to take advantage of the mature processing tools of thesilicon microelectronics technology where lateral dimensions on amicrometer scale may readily be achieved. Thus, a silicon mold isfabricated by high-aspect ratio etching of a silicon substrate for forman array of pores. This array is subsequently oxidized to provide a lowrefractive index layer in contact with each individual scintillatorblock, formed by melting a scintillating material into the pores.

A scintillator device according to the present invention presents astructure based on light guiding of secondarily produced scintillatingphotons in a pixel detector in conjunction with, for instance, a CCD ora CMOS pixel detector. The structure according to the invention presentsa matrix having deep pores created by thin walls presenting a porespacing appropriate to the image detector in use, and may utilize areflective layer on the walls of the matrix to increase light guidingdown to the image detector chip.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 illustrates a silicon CCD pixel detector for direct irradiationby X-rays;

FIG. 2 illustrates a pixel detector as of FIG. 1 but provided with athin scintillator for increasing its efficiency for X-ray radiation;

FIG. 3 illustrates a pixel detector as of FIG. 1 provided with a thickscintillator for further increasing the efficiency for X-ray radiation,but then loosing resolution;

FIG. 4 illustrates a CCD pixel detector using a thick pixel scintillatorresiding inside pores formed in a matrix material according to thepresent invention for maximum sensitivity and maintained resolution;

FIG. 5 is a more detailed view of the structure forming pores forincreasing the efficiency of a CCD pixel detector; and

FIG. 6 is an enlargement of a portion of a pore indicating an extralayer of silicon oxide for improving the wall reflecting properties.

DETAILED DESCRIPTION

General Features

The most developed etching techniques exist for silicon processing.According to the present application a grid is created by etchingrectangular holes in a silicon wafer. The holes can be etched to acertain depth or go all the way through the wafer. The holes are thenfilled with scintillating material.

The performance of such a device strongly depends on how well the holesare filled, the transparency of the scintillator and the reflectionproperties of the walls of the hole.

The present X-ray pixel detector concept is for clarity compared toexisting technology demonstrated in FIGS. 1 to 4. FIG. 1: A standardsilicon CCD arrangement has a very low efficiency for X-ray photondetection, normally of the order of a few per cent. This is because thepenetration depth of X-ray photons, at energies of the order 40 keV, isof the order of 1 cm in silicon and thus the fraction absorbed withinthe active CCD layer is small

The efficiency will preferably be increased significantly by using ascintillating material emitting a large number of visible photons forevery absorbed X-ray photon as is indicated in FIG. 2. Typicalabsorption lengths for X-ray photons, at energies of the order 40 keV,are several 100 μm. As already mentioned a layer of the order 300 μm ofCsI is needed to absorb about 80% of the X-ray photons. Thus, for thickscintillating films as indicated in FIG. 3, almost al X-ray quanta maybe absorbed, which results in a high efficiency detector. However, thetrade-off is resolution, which becomes much worse as the scintillatoremits photons isotropically, such that nearby pixels will also detect asignificant number of photons. An alternative route is to use a thinscintillating film (of about same thickness as a pixel size) asindicated in FIG. 2, but at the expense of a much lower efficiency.

Finally, in FIG. 4 is shown the concept of the invention resulting inboth high efficiency and high resolution. Here, a thick scintillator isused which has been patterned into pixels corresponding to the size ofthe pixels of the detector, e.g. a CCD, in such a way that thescintillator pixels also serve as light guides which confine the emittedphotons to the same pixel element only. Thus, no cross talk betweenpixels takes place and, depending on the pixel thickness (lengthperpendicular to the CCD sauce) up to 100% of the incoming X-ray photonsmay be absorbed. However, in order to achieve a large effectivedetection area the spacing between pixels must be short, e.g. for atypical 44 μm pixel size a 4 μm gap between pixels results in −82%efficiency due to the ‘dead area’ in between pixels. Clearly, tominimize cross-talk pixels may be reflection coated or the medium inbetween should be highly absorbing.

The fabrication of pixels having a thickness of 300 μm and a gap ofabout 4 μm from a scintillating material is not an easy task. Thepresent invention therefore benefits from the mature silicon processtechnology using a silicon matrix wherein corresponding pores have beenfabricated and successively filled with a scintillator material. Thefabrication technology involves more or less standard siliconfabrication technologies such as Deep Reactive Ion Etching (DRIE),oxidation and/or metallisation. A schematic drawing of the structure isshown in FIG. 5 where 3 pixels are displayed together with a close-up ofthe wall structure between adjacent pixels being demonstrated in FIG. 6.In essence, the structure contains three different materials to providethe light-guiding effect the processing of which is accomplished oneafter the other:

Silicon Pore Matrix

The silicon pore matrix of the present application may be fabricatedusing two different techniques: Deep Reactive Ion Etching (DRIE) orElectrochemical etching. DRIE is now an established technique andseveral hundred μm deep pores may be fabricated. It has been found thatit is possible to make, for instance, 40×40 μm square-formed pores witha wall thickness of 3-4 μm (representing −80% active area) and with adepth of a few hundred μm. A similar structure may be formed byelectrochemical etching of silicon starting from pore initiation conesmade by conventional lithography and non-isotropic etching.

Wall Reflection Layer

Scintillating materials usually have an index of refraction (for CsIn=1.79) which is significantly lower than that of silicon (n=3.4). Thus,the major fraction of scintillating photons impinging on the pore wallswill penetrate into the silicon (Si) matrix unless some reflectioncoating of the pore walls has been provided. Therefore, this simplestructure will have much lower efficiency since no light guiding exists.In the silicon matrix the light will be quickly absorbed due to the highabsorption coefficient for visible light in silicon. However, note thatthis is a clear advantage of the present invention, as opposed toseveral of the structures cited in the Background paragraph, as ittotally eliminates any cross talk between pixels.

To provide light guiding a reflecting layer must be introduced at thewalls. This may be accomplished either by oxidation or by coating with ametal layer. Whereas silicon dioxide is much more stable duringsubsequent processing, metal coating provides better reflection. In thecase of an oxide, a total reflection results whenever the entrance angleis larger than the result of the expression arcsin(n2/n1), where n2 andn1 represents a respective refractive index The reflection results in alight-guiding cone propagating upwards and downwards in the pore, seeFIG. 5. The difference to a metal-coated pore (where all light would beguided in the pore) is, however, not that large as light rays impingingon the walls close to normal incidence correspond to very long pathlengths before reaching the image detector cell and thus absorption ismore likely.

Finally, a reflecting layer at the bottom of the pore (or at the topsurface for a pore structure, which is transparent) is desirable inorder to redirect and collect photons emitted in the upward direction.

Filling With Scintillating Material

Filling of the pores with scintillating material is a crucial step.Extensive tests have proved that filling of the pores with scintillatingpowder without melting does not yield an operational device structure.This is because grain boundary scattering of the light results in a veryshort penetration distance. An index-matched fluid could possiblycircumvent this problem but the low effective density of thescintillator powder (large unfilled fraction) would then demand verydeep pores.

Due to this fact our invention involves melting of the scintillatingmaterial to form single or polycrystalline blocks of scintillatormaterial within each pore. For this purpose we have used CsI as asuitable material as it does not decompose upon melting. The melting andfilling should be carried out in a vacuum to reduce problems with airbubbles in the pores, which significantly affects efficiency and thelight guiding ability of the pores.

In summary, the present invention is based upon light guiding ofsecondarily produced scintillating photons in a pixel detector inconjunction with, for instance a CCD camera or a corresponding device.The three ingredients of the preferred embodiment of the structure are:

a) A matrix with deep pores, thin walls and a pore spacing appropriateto the image detector chip in use

b) A reflective layer on the walls to increase light guiding down to theimage detector chip

c) A suitable scintillating material which is melted into the pores toform a single scintillating block in order to eliminate grain-boundaryscattering

In addition, the invention concerns a suitable fabrication method tothis structure in an efficient way suitable for mass production.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

What is claimed is:
 1. A method for fabricating a structured highresolution scintillating device based on light guiding of secondaryproduced scintillating photons for use in an X-ray pixel detector devicewith an image detector chip (1), characterized by the steps offabrication of a silicon pore matrix (8) presenting a pore spacing (10)corresponding to the image detector pixel size (2), by utilizing siliconetching techniques such as deep reactive ion etching, electrochemicaltechniques or other techniques providing high-aspect ratios such thatthin pore walls of thickness reaching down to a few micrometers will bemaintained for an active detection area optimization; using the siliconpore matrix (8) as a mold when melting a scintillator material into thepores to form in each pore a single scintillating block in order toeliminate grain-boundary scattering of scintillating photons.
 2. Themethod according to claim 1, characterized by the further step of, afteretching but before molding, depositing a metallic reflective layer inthe pores.
 3. A method of fabricating a high resolution scintillatingdevice for an X-ray pixel detector, comprising the steps of: fabricatinga silicon pore matrix having plural pores corresponding to locations ofpixels in the X-ray pixel detector, the plural pores being formed byetching a silicon substrate; melting a scintillating material into theplural pores of the silicon pore matrix to form in each of the pluralpores a single scintillating block; and providing, after the fabricatingstep but before the melting step, a reflection layer for light guidingby oxidation of the silicon pore matrix or by deposition of any layerhaving a resulting refractive index being lower than that of thescintillating material.
 4. A scintillating device for simultaneouslymaintaining resolution and increased sensitivity for X-ray radiation inan imaging arrangement, characterized by utilization of a fabricationmethod producing a silicon pore matrix (8) presenting a pore spacing(10) corresponding to an image detector pixel size (2), the pore matrixhaving deep pores (10) presenting thin walls of a thickness reachingdown to a few micrometers to create a pore spacing corresponding to thepixel size (2) of an image detector chip (1), the pore matrix (8)further containing scintillating material which is melted into the pores(10) to form in each pore a single scintillating block in order toeliminate grain-boundary scattering of scintillating photons.
 5. Thedevice according to claim 4, characterized by a reflective layer (12)onto the thin walls of the matrix to increase light guiding down to theimage detector chip (1).
 6. The device according to claim 4, furthercomprising a reflection layer on walls of the pores, the reflectionlayer being one of an oxidation of the silicon pore matrix and a layerhaving a refractive index lower than a refractive index of thescintillating material.
 7. A method of fabricating a high resolutionscintillating device for an X-ray pixel detector, comprising the stepsof: forming plural pores in a silicon substrate to form a silicon porematrix; and melting a scintillating material into the plural pores ofthe silicon pore matrix to form a scintillating block in each of theplural pores.
 8. The method of claim 7, further comprising the step ofproviding a reflection layer on walls of the pores by oxidizing thesilicon pore matrix in the pores.
 9. The method of claim 7, furthercomprising the step of providing a reflection layer on walls of thepores, the reflection layer having a refractive index lower than that ofthe scintillating material.
 10. The method of claim 7, wherein theplural pores correspond to locations of pixels in the X-ray pixeldetector.
 11. The method of claim 7, wherein the plural pores are spacedmore closely than pixels in the X-ray pixel detector.